Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
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FXUS62 KRAH 261430
Area Forecast Discussion
National Weather Service Raleigh NC
1030 AM EDT MON SEP 26 2016
A quasi-stationary front across southwestern NC will return north as a
warm front today as a cold front approaches from the west. The cold front
will move very slowly southeast through the area Tuesday and into Wednesday.
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 1030 AM Monday
High pressure off the New England coast extends into east-central NC
this morning, with an east-southeasterly flow banking moisture up
against the mountains and ceilings varying from below 1000ft in the
west to 3000-4000 ft in the east. Persistent convection around the
Charlotte Metro area has shown signs of waning in the past couple of
hours, likely owing to a relaxation of the nocturnal LLJ and weaker
isentropic accent. This precip, along with addition showers
expected to trigger within an instability axis running up the
foothills this afternoon, will have less moisture and forcing to
work with as the mid-level flow carries it eastward. Thus, we will
run with POPs varying from 30-40% in the western Piedmont to 20%
Temperatures continue to be a tough call, especially in the western
Piedmont, as a southeasterly wind has developed, but the moisture
(stratus) is somewhat trapped horizontally but the mountains and
vertically by a weak CAD-like inversion (per 12z GSO raob). However,
850/925mb temps are around 16-18C over the southern and western
portions of the Piedmont, so the airmass will support strong warming
if/when clouds breaks. We`ve blended in some HRRR and LAMP guidance
to help with the uncertainty and possible gradient, which yields mid
70s northeast to mid 80s southeast.
Sfc cold front is forecast to slip east of the mountains, but remain
west of the forecast area through daybreak Tuesday. Upper support is almost
non-existent, so expect convection to be tied closed to the low-level
moisture convergence along the front and thus expect bulk of shower
activity to remain across the NC Foothills and Western Piedmont.
Lows in the mid 60s to lower 70s.
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.SHORT TERM /TUESDAY AND TUESDAY NIGHT/...
As of 415 AM Monday...
Low-level frontal zone stretched across central NC will slow down,
potentially temporary stall across the area as the closed upper low
over the Great Lakes and attendant deep upper trough begins to digs
south into the Ohio Valley and southern Appalachians. Axis of 1.6-
1.7" PWAT attendant to the front will be centered across the area
Tuesday afternoon and Tuesday night with numerous showers and
scattered thunderstorms expected to move slowly south-southeastward
across the area. Weak mid-level lapse rates should again limit
destabilization and thwart severe threat. We could however see some
locally heavy rain and minor flooding with models suggesting
rainfall amounts widespread rainfall amounts of a half inch to an
inch, with locally higher amounts of 1 to 1.5 inches possible. The
front should settle slowly across eastern portions by late Tuesday
evening/night with rain chances beginning to taper off from nw to se
by daybreak Wednesday.
Highs in the mid 70s northwest to lower 80s se. Lows Tuesday night
60 to 67.
.LONG TERM /Wednesday through Sunday/...
As of 345 AM Monday...
Forecast confidence remains below average due to an atypically
complex synoptic pattern expected to evolve over North America
mid/late week. The 00Z GFS/ECMWF both indicate that an upper level
low amplifying over the Great Lakes early this week will dig
southward through the OH valley and cut-off in vicinity of the TN
Valley/Mid-Atlantic through mid/late week as an expansive upper
level ridge builds over the eastern 2/3 of North America, though
disagreement persists w/regard to the precise evolution of the cut-
off low. Broadly speaking, long range guidance supports above normal
chances for cloud cover/precipitation and slightly below normal
temperatures mid/late week, though further details (i.e. precip
amount/timing and perhaps severe wx potential) remain difficult to
ascertain at this time. -Vincent
.AVIATION /12Z MONDAY THROUGH THURSDAY/...
As of 740 AM Monday...
Southerly moist upglide atop a quasi-stationary frontal boundary that
extends across the western piedmont will support a chance of showers
primarily over the western and central piedmont this morning. The front
will retreat northeastward as a warm front between 15 to 21z. IFR to MVFR
ceilings over the piedmont will lift to VFR with the passage of
the warm front.
Moisture pooling ahead of a cold front moving into the
mountains will also support a small chance of showers and possibly
a thunderstorm late in the afternoon/early evening with rain chances
increasing overnight with sub-VFR ceilings as the front approaches
the NW Piedmont.
Looking beyond the 24 TAF Period: A cold front will move very slowly
se through the area Tuesday and into the day on Wednesday. Showers
and storms with associated periods of sub-VFR conditions are
expected to accompany the front. Conditions are expected to improve
to VFR from northwest to southeast Wednesday-Wednesday night as
drier air filters into the area.
During midweek, a rather unsettled pattern is expected to evolve as
a deep upper low sags southward from the Great Lakes into the